Set in modern Japan, the narrative focuses on how ten years prior to the events of series, a mysterious spatial anomaly known as "Heaven's Gate" appeared in South America, shortly followed by the opening of "Hell's Gate" in Tokyo altering the sky and wreaking havoc on the landscape.

During this time, people possessing various special abilities emerged, each capable of different supernatural feats—these are known as contractors.

The series follows the exploits of agents Hei and his comrades of the Syndicate organization operating in a post-apocalyptic Tokyo and their mission to uncover the mysteries of the Hell's Gate.

[3] The series was broadcast on MBS between April 6 and September 28, 2007, and aired later on HBC, CBC, TBS and Animax.

[5][6] The ninth DVD volume also included an unaired 26th original video animation episode.